1. Field of the Invention
This invention relates to a vehicle headlamp having as its light source a metal-halide discharge lamp comprising an inner envelope and a surrounding light-transmitting shroud integral with the inner envelope. The invention also relates to a discharge lamp per se of this type.
2. Background of the Disclosure
In U.S. Pat. No. 4,935,668--Hansler et al, there is disclosed and claimed a type of metal-halide lamp that comprises (i) a quartz inner envelope within which an electric discharge, or arc, is developed and (ii) a tubular glass or quartz shroud surrounding the inner envelope and spaced therefrom along a portion of the shroud length. The tubular shroud is sealed at predetermined locations along its length to the inner envelope, and the space between the shroud and the inner envelope constitutes a sealed chamber that is either evacuated or gas filled, depending upon the particular application of the lamp. The shroud and the sealed chamber serve a number of important functions which are discussed in detail in the patent. Generally speaking, one of these functions is to make the temperature of the inner envelope higher and more uniform during lamp operation, and another is to keep the shroud relatively cool in comparison to the inner envelope during lamp operation.
The ability to accomplish the results desired from the shroud and the vacuum chamber or gas chamber depends materially upon the nature of the joints or seals formed between the shroud and the inner envelope. A discharge lamp, being a diffuse light source, inherently produces a headlamp beam with lower seeing-to-glare ratio (SGR) than a filament lamp. Further, when a shroud is added to the discharge lamp, the light reflected and refracted from the shroud can significantly add to the glare light, reducing the SGR to undesirable levels. The refracted light comes primarily from the junctions between the bulbous light-emitting region and the cylindrical legs of the shroud, as well as from the shroud-to-arctube seal regions.
In European Patent Application Publication 0 465 083A2--Biel et al, which is assigned to the assignee of the present invention, there is disclosed and claimed a type of seal that can be advantageously used in these locations since, among other things, it is a high quality seal that can be quickly made with very little heat, with a low risk of damaging inner envelope components, and with little change in the thermal characteristics of the lamp in the seal region should there be slight variations in the process of making the seal. This seal comprises a disk-shaped enlargement formed in a tubular portion of the inner envelope by first heating a localized region of the tubular portion to its softening point and then subjecting this region to an abrupt, longitudinally-applied compressive force that drives the softened quartz material radially outward into a disk formation (which we refer to herein as a "maria"). Then the disk-shaped enlargement, or maria, is positioned in alignment with a predetermined surrounding portion of the shroud slightly radially spaced therefrom, following which the predetermined surrounding shroud portion is heated and thus softened and caused to collapse about the outer periphery of disk-shaped enlargement, thereby forming the desired seal at the outer periphery of the disk-shaped enlargement. This type of seal we refer to herein as a "maria seal".
While a maria seal has many advantages, it is subject to the disadvantage that light passing therethrough tends to be scattered. Being at the outer periphery of a disk-shaped enlargement, which typically has a relatively large diameter, the maria seal has been located in prior discharge lamps in a position where it would increase the amount of scattered light in the utilized light output from the discharge lamp. The effect of this in a headlamp system that includes such a discharge lamp is to increase the amount of glare present in the headlamp beam, which is a decidedly undesirable effect. Our invention, in one of it aspects, is concerned with overcoming this disadvantage while retaining most of the advantages of a large-diameter maria seal. Other aspects of the invention are pointed out in the last two paragraphs of the following "Summary".